U.S. Pat. No. 4,152,947 discloses transmission control method wherein the transmission ratio of the transmission is set or adjusted by means of a hydraulic valve having an internal pilot spool or pilot sleeve which on one end is loaded or biased by a fluid pressure supplied by means of the Pitot tube in dependence on the input rotational speed, i.e. in proportion to the input rotation speed. On the other end the pilot sleeve is loaded or biased by a spring which is compressed in dependence on (proportional to) an external control signal such as, in case of a motor vehicle transmission, the inlet manifold pressure or the position of throttle pedal or throttle valve. To achieve this function a control member compressing the spring is connected for instance to a vacuum-operated cylinder or via a cam to the throttle pedal or throttle valve.
In the above known method the transmission ratio of the transmission is always set or adjusted in such a way that the engine speed (input speed of the transmission) assumes a given value, depending on and proportional to the external control signal, e.g. on the position of the throttle pedal, and naturally in so far this is possible within the ratio range of the transmission.
In the case of a motor vehicle, when one has to accelerate with a given acceleration, a certain motor power will be required. The transmission will then shift in such a way that the engine speed optimum for said motor power is set and this speed will be maintained during the acceleration as long as the parameters defining the required engine speed do not change. In certain cases it has been found that it is desirable that the input speed (the engine speed) depends not only on an external control signal but also in part, e.g. on a required output power and on specific conditions of the drive motor.
In a motor vehicle the drivability may be improved by a control whereby in case of acceleration (increasing vehicle speed) the engine speed increases at for the rest constant parameters.